This application claims priority on Japanese Patent Application No. 2006-232568 filed in Japan on Aug. 29, 2006, and Japanese Patent Application No. 2007-057393 filed in Japan on Mar. 7, 2007, the entire contents of which are hereby incorporated by reference.
The present invention relates to an image forming apparatus, and more specifically relates to an image forming apparatus compatible with a high speed device that transports more than 100 sheets per minute to a transfer portion.
In recent image forming apparatuses, space-saving vertical transport-type image forming apparatuses have become mainstream in order to reduce installation space. More specifically, this sort of apparatus has a structure in which a plurality of paper feed cassettes are disposed at multiple levels in the lower portion of the apparatus body, and a transfer portion and a fixing portion are disposed in the upper portion of the apparatus body. With such a structure, transport paper fed from a paper feed cassette is temporarily transported upward, then curved about 90 degrees at the front of the transfer portion to change to transport in the horizontal direction, and then transported toward the transfer portion.
FIG. 22A shows the configuration of a paper transport path at the front side of a transfer portion.
In a paper transport path 28 disposed in a curved shape as described above, pairs of transport rollers R31 and R32, pre-registration rollers R41 and R42, and registration rollers R51 and R52 are disposed in order in a paper transport direction S. The registration rollers R51 and R52, at a distance of about 50 mm, face a nip portion N1, which is a contact portion of an electrostatic latent image carrier (a photosensitive drum) 14 and a transfer roller 17a. 
FIG. 23 is a chart that shows the operation timing of the registration rollers R51 and R52 and the pre-registration rollers R41 and R42.
When paper P is transported through the paper transport path 28 to the registration rollers R51 and R52, and the leading edge of the paper makes contact with a nip portion N5 of the registration rollers R51 and R52, first the registration rollers R51 and R52 stop at a time t1, and the pre-registration rollers R41 and R42 stop at a time t2 that is slightly after the time t1. Due to stoppage according to this time difference, the paper P being held sandwiched by the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 is kept in a state bowed in a slight curve, as shown in FIG. 22A.
Afterward, at a timing t3 when an image forming position (for example, the leading edge) of the paper P is caused to coincide with an image writing position (for example, the leading edge) of image information that has been made visible on the photosensitive drum 14, the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 start transport of the paper P again. At this time, because the paper P is slightly bowed, the feed-out timing of the leading edge of the paper P from the registration rollers R51 and R52 can be made uniform in the widthwise direction of the paper. Thus tilting of the paper P is prevented, eliminating distortion of the formed image in the widthwise direction of the paper P. More specifically, with respect to paper P that has been temporarily stopped by the registration rollers R51 and R52, during this stoppage, fine adjustment of the paper transport position (for example, the position of the leading edge) in a direction parallel to a paper transport direction S, adjustment of the center position of the paper P in the transport path (see paper transport direction S) during transport, and tilting of the paper P in the transport path (see paper transport direction S) during transport, and the like are corrected by the registration rollers R51 and R52.
Incidentally, in recent image forming apparatuses, accelerated print processing speed relative to conventional image forming apparatuses has been sought. For example, conventionally, an image forming apparatus capable of 60 sheets per minute (in the case of A4 sideways transport) or greater was deemed to be a high speed device, but recently, an image forming apparatus capable of 80 sheets per minute or greater is referred to as a high speed device, and moreover, development of image forming apparatuses capable of not less than 100 to 120 sheets per minute is progressing.
In this sort of image forming apparatus, it is necessary to achieve an improvement in print quality when transferring a toner image on a photosensitive drum onto paper. An important factor for insuring this print quality is increasing the precision of matching the image writing position of the image information that has been made visible on the photosensitive drum 14 with the image forming position of the paper P stopped by the registration rollers R51 and R52.
As technology for improving the precision of position-matching in this manner, a paper transport apparatus is proposed in JP 2003-248410A and JP 2003-330334A.
JP 2003-248410A discloses an apparatus that detects a length of displacement in a direction perpendicular to the paper transport direction, and performs control so as to correct the image writing position. JP 2003-330334A discloses an apparatus that detects a length of displacement in a direction perpendicular to the paper transport direction, and performs control so as to continue image forming when that displacement length does not exceed a prescribed value, and temporarily stop paper transport when that displacement length exceeds the prescribed value.
However, in recent high speed devices, due to acceleration of the paper transport speed, during consecutive printing of a plurality of sheets, adjustment of paper transported subsequent to the first sheet is difficult to perform in a timely manner. More specifically, with respect to the first sheet of printing, there is time to spare in the initialization process of the apparatus, so by making the timing for feed of paper from a paper feed portion earlier, it is possible to insure time to spare for adjustment of the image writing position. However, the timing for transport of the second and subsequent sheets of paper depends on the print processing speed, i.e., the transport speed, and so before the leading edge of the second and subsequent sheets of paper makes contact with the nip portion of the registration rollers, writing of image information onto the photosensitive drum has started, and therefore no time to spare is available for adjusting the desired position on the paper where an image will be formed.
Consequently, in order to address the problems described above, it is an object of the present invention to provide an image forming apparatus in which a desired image forming position on paper and a position of image writing to an image carrier can be matched together with good precision even in a high-speed device.